Microporous Crystalline Materials
Zeolites are microporous crystalline materials of variable composition characterized by a TO4 tetrahydra crystalline lattice (wherein T represents atoms in the formal oxidation state of +3 or +4, such as for example Si, Ti, Al, Ge, B, Ga which all share their vertexes giving rise to a three-dimensional structure containing channels and/or cavities of molecular dimensions. When some of the atoms T have an oxidation state lower than +4, the crystalline lattice formed has negative charges which are compensated by the presence of organic or inorganic cations in the channels or cavities. Organic molecules and H2O can also be located in these channels and cavities, so in general, the chemical composition of zeolites can be represented by the following empirical formula:
X(M1/nXO2):yYO2:SiO2
wherein M is one or several organic or inorganic cations with charge +n; X is one or several trivalent elements; Y is one or several tetravalent elements, generally Si; and R is one or several organic substances. Although the nature of M, X, Y and R and the values of x, y, z and w can, in general, be varied by means of post-synthesis treatments, the chemical composition of a zeolite (just as it is synthesized or after calcination thereof) has a range characteristic of each zeolite and its preparation method.
On the other hand, a zeolite is also characterized by its crystalline structure, which defines a system of channels and cavities and gives rise to a specific X-ray diffraction pattern. In this way, zeolites are differentiated from each other by their range of chemical composition plus their X-ray diffraction pattern. Both characteristics (crystalline structure and chemical composition) also determine the physicochemical properties of each zeolite and the applicability thereof in different industrial processes.
The present invention refers to a microporous crystalline material of zeolitic nature named ITQ-3, 5to its method of obtainment and to its applications.
The material is characterized by its chemical composition and its X-ray diffraction pattern. In its anhydrous and calcined formed, the chemical composition of ITQ-3 may be represented by the empirical formula:
X(M1/nXO2):yYO2:SiO2
wherein x has a value lower than 0.15; it may be equal to zero; and y has a value lower than 0, 1; it may be equal to zero; M is H+ or an inorganic cation of charge +n; X is a chemical element with oxidation state (Al, Ge, B, Cr) and Y is a chemical element with oxidation state +4 (Ti, Ge, V), when x=0 and y=0 the material can be described as a new polymorphous of silica of microporous nature. In the preferred embodiment of the present invention, ITQ-3 has the composition, in a calcined and anhydrous state
x(HXO2):SiO2
wherein X is a trivalent element and x has a value lower than 0.1 and may be equal to zero, in which case the material may be described by means of the formula SiO2. However, it is possible, in terms of the synthesis method and the calcination or subsequent treatments, the existence of defects in the crystalline lattice, manifested by the presence of Sixe2x80x94OH groups (silanols). These defects have not been included in the above empirical formulae. In a preferred embodiment of the present invention, ITQ-3 has a very low concentration of this type of defect (silanol concentration lower than 15% with respect to the total Si atoms, preferably lower than 6%, measured by nuclear magnetic resonance spectroscopy of 29Si in spinning angle).
The X-ray diffraction pattern of ITQ-3 just as it is synthesized as obtained by the powder method using a variable divergence slit and the Cu Kxcex1 radiation, is characterized by the following values of 2xcex8 angles and relative intensities (I/IO):
The positions and relative intensities of the peaks depend to a certain degree on the chemical composition of the material (the pattern represented in Table I refers to the material whose lattice is exclusively comprised of silicon oxide, SiO2 and synthesized using a quaternary ammonium cation as a structure-directing agent). The relative intensities may also be affected by phenomena of preferred orientation of the crystals, produced during preparation of the sample, while the precision in the interplanar spacing measurement depends on the quality of alignment of the goniometer. Moreover, calcination can yield significant changes in the X-ray diffraction pattern, due to the removal of organic compounds retained during synthesis in the zeolite pores, so that Table II represents the X-ray diffraction pattern of ITQ-3 of calcined ITQ-3 of composition SiO2 is represented:
The present invention also refers to the method of preparation of ITQ-3. This comprises thermal treatment at temperatures between 80 and 200xc2x0 C., preferably between 130 and 180xc2x0 C., of a reaction mixture that contains a source of SiO2 (such as, for example, tetraethylorthosilicate, colloidal silica, amorphous silica), an organic cation in hydroxide form, preferably N,N-dimethyl-6-azonium-1,3,3-trimethylbicyclo(3.2.1)octane (I) hydroxide, hydrofluoric acid and water. Alternatively, it is possible to use the organic cation as a salt (for example, a halide, preferably chloride) and to substitute hydrofluoric acid by a fluoride salt, preferably NH4F. The reaction mixture is characterized by its relatively low ph less than 12, preferably less than 11, and which may also be neutral or slightly acidic. 
Optionally, it is possible to add a source of another tetravalent element Y and/or trivalent element X, preferably Ti or Al. The addition of this element may be done before heating the reaction mixture or in an intermediate time during said heating. Occasionally, it may be convenient to add also in a certain time during the preparation of ITQ-3 crystals (up to 15% by weight with respect to the total inorganic oxides, preferably up to 10% by weight) as cryst6allizer promoters (seeding). The composition of the reaction mixture in oxide form responds to the general formula
RR2O:aHF:xHXO2:yYO2:SiO2:wH2O
wherein X is one or several trivalent elements, preferably Al; Y is one or several tetravalent elements; R is an organic cation, preferably N,N-dimethyl-6-azonium-1,3,-trimethyl-6-bicyclo(3.2.1.)octane, and the values of r,a,x,y and w are in the ranges
R=0.05-1.0, preferably 0.1-0-75
A=0-1.5, preferably 0.1-1.5
X=0-0.15
Y=0-0.1
W=3-100, preferably 5-50, more preferably 7-50
The thermal treatment of the reaction mixture may be done in static or with stirring of the mixture. Once the crystallization is finished the solid product is separated and dried. Subsequent calcination at temperatures between 400 and 650xc2x0 C., preferably between 450 and 600xc2x0 C., produces the decomposition of the organic residue occluded in the zeolite and renders the free zeolitic channels.
This method of synthesis of ITQ-3 zeolite has the peculiarity that it does not require introduction of alkali cations in the reaction medium. As a consequence the organic cation R is the only cation that balances the lattice charges when the zeolite contains a trivalent element in its crystalline lattice. Therefore, simple calcination to decompose the organic cation leaves the zeolite in acid form, without the need to resort to cation exchange processes. Besides, the absence of alkali cations in the reaction mixture allows synthesis of the material containing elements such as Ti(IV), which would not be possible to introduced in the lattice in the presence of these cations (see, for example, M. A. Camblor, A. Corma, J. Pxc3xa9rez-Pariente, Zeolites, vol. 13, 82-87, 1993). Thus, once calcined the material has the general formula
X(HXO2):yYO2:SiO2
wherein x is lower than 0.15, and may be equal to 0; y is lower than 0.1, and may also be zero; X is a chemical element with oxidation state of +3 and Y is a chemical element with an oxidation state of +4.
The crystalline material of the present invention may be used in several applications, such as, for example, in processes for the separation of linear and branched paraffin compounds. Hence, a mixture of isobutane and n-butane or isopentane and n-pentane may be enriched in the more branched isomer by selective adsorption of the linear paraffin by the microporous material of the present invention. Said material is particularly suitable for use in this type of process due to its high adsorption capacity (micropore volume determined by adsorption of N2=0.23 cm3/g) and its small pore size (maximum opening xe2x89xa65.5 xc3x85, determined by adsorption of Ar, using the Horvath-Kawazoe formalism).
Likewise and preferably using the pure silica polymorph, it is possible to separate by selective adsorption the n-olefins of mixtures containing normal and isoolefins, enriching the final stream in isoolefins. In general, this material would allow the separation of organic compounds, which may or may not contain heteroatoms, with sizes smaller than 5-5.5 xc3x85 present in mixtures also containing organic compounds of larger sizes. Due to the hydrophobic characteristics of the silica polymorph, ITQ-3 would permit selective adsorption of organic compounds with kinetic diameter smaller than 5-5.5 xc3x85 present in polar media, such as, for example, in aqueous media.
From the view point of its use as a catalyst, this material, when prepared in the acid form and which may or may not contain supported transition metals such as Pt, Pd or Ni, allows the selective cracking and hydrocracking of linear alkanes with respect to the branched ones or to larger hydrocarbons, thus being adequate as a catalyst or catalytic cracking additive and as a catalyst in the xe2x80x9cselectoformingxe2x80x9d type process that involves hydrocracking of the stream coming from the reformed unit for the purpose of removing n-paraffins.
Likewise, ITQ-3 gives good results as an alkane and alkene catalyst for the purpose of producing high yields of ethylene, propylene and butene, thus being suitable as a catalyst for production processes of short olefins by catalytic steam cracking. Moreover, its possibilities of selectively cracking linear paraffins makes it a good catalyst for dewaxing processes. Finally, this material is a good catalyst in processes of transformation of methanol into olefins.